Asymmetric segmented channel transistors

ABSTRACT

Structures, layouts and methods of forming integrated circuits are described. In various embodiments, the current invention includes an asymmetric segmented transistor. The asymmetric segmented transistor includes a source region and a drain region disposed within an active region, a floating source/drain region disposed within the active region, a first channel region disposed in the active region between the source region and the floating source/drain region, the first channel having a first length and a first width. A second channel region is disposed in the active region between the drain region and the floating source/drain region, the second channel having a second length and a second width. A first gate dielectric overlies the first channel region and a second gate dielectric overlies the second channel region. A gate line overlies the first gate dielectric and the second gate dielectric.

This application is a divisional of U.S. patent application Ser. No. 13/275,898 filed on Oct. 18, 2011, which is a divisional of U.S. patent application Ser. No. 12/036,911 filed on Feb. 25, 2008 and issued as U.S. Pat. No. 8,067,287, both of these applications are incorporated herein in entirety by reference.

TECHNICAL FIELD

This invention relates generally to electronic devices, and in particular embodiments to asymmetric segmented channel transistors.

BACKGROUND

Semiconductor devices are used in a large number of electronic devices, such as computers, cell phones and others. One of the goals of the semiconductor industry is to continue shrinking the size and increasing the speed of individual devices. Every generation of semiconductor devices is smaller than the previous generation by about 50%, thus reducing the cost per device significantly.

A key challenge in scaling semiconductor devices arises due to the increase in leakage currents at short channel lengths. Halo implants are used to mitigate this increase leakage arising from short channel effects. However, use of halo implants increases the channel doping as well counter-dopes the source and drain junctions. Both these effects increase the effective resistance of the transistor. However, short channel devices compensate some of this loss by gate length scaling accompanying the technology development. However, longer gate length transistors built on such short channel device technologies do not enjoy this advantage and are hence degraded. This is often not a problem in digital technologies that use only the short gate length devices.

Analog circuits typically employ longer gate length devices and cannot take advantage of these aggressively scaled transistors. For example, these short channel digital devices have lower analog intrinsic gain as well as increased gate leakages. Due to cost contraints, analog technologies utilize the CMOS technologies developed for digital applications. As the long gate lengths devices are also degraded, many analog applications require individually tailored technology flows, considerably increasing production costs.

Hence, improved methods, structures of forming and designing analog devices are needed that are compatible with existing CMOS technologies.

SUMMARY OF THE INVENTION

These and other problems are generally solved or circumvented, and technical advantages are generally achieved, by embodiments of the present invention which provide structure, methods and circuits comprising asymmetric devices.

Embodiments of the invention include methods, structures and layouts for asymmetric segmented transistors in integrated circuits. In accordance with an embodiment of the present invention, a mixed signal integrated circuit includes an asymmetric segmented transistor comprising a source region and a drain region disposed within an active region, a floating source/drain region disposed within the active region, a first channel region disposed in the active region between the source region and the floating source/drain region, the first channel having a first length and a first width. A second channel region is disposed in the active region between the drain region and the floating source/drain region, the second channel having a second length and a second width. A first gate dielectric overlies the first channel region and a second gate dielectric overlies the second channel region. A gate line overlies the first gate dielectric and the second gate dielectric.

The foregoing has outlined rather broadly the features of embodiments of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of embodiments of the invention will be described hereinafter, which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and specific embodiments disclosed may be readily utilized as a basis for modifying or designing other structures or processes for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:

FIG. 1 illustrates the key analog device parameters for different gate length transistors;

FIG. 2 illustrates the key analog device parameters for a given device or gate length;

FIG. 3, which includes FIGS. 3 a-3 c, illustrates a structural embodiment of the invention, wherein FIGS. 3 a and 3 b illustrate top cross sections and FIG. 3 c illustrates a cross sectional view;

FIG. 4, which includes FIG. 4 a-4 c, compares the key analog device parameters from the structural embodiment described in FIG. 3 for use in high precision analog circuits;

FIG. 5 compares the key analog device parameters from the structural embodiment described in FIG. 3 for use in high performance analog circuits;

FIG. 6, which includes FIGS. 6 a-6 f, illustrates a top view of structural embodiments of the invention;

FIG. 7 is a flow chart of a method of forming the analog devices according to an embodiment of the invention;

FIG. 8, which includes FIGS. 8 a-8 e, illustrates cross sections of the device during various stages of the manufacturing illustrated in flow chart 7;

FIG. 9, which includes FIGS. 9 a and 9 b, wherein FIG. 9 a illustrates an embodiment of a mixed signal chip comprising analog devices and logic devices and FIG. 9 b illustrates the threshold voltages used in the analog and logic devices in accordance with embodiments of the invention;

FIG. 10, which includes FIGS. 10 a and 10 b, wherein FIG. 10 a illustrates an embodiment of a mixed signal chip comprising analog devices and logic devices, the analog devices comprising at least one device with multiple segments, and FIG. 10 b illustrates the threshold voltages used in the analog and logic devices in accordance with embodiments of the invention;

FIG. 11, which includes FIGS. 11 a-11 c, illustrates two-stage CMOS amplifier circuits incorporating embodiments of the invention; and

FIG. 12, which includes FIG. 12 a and FIG. 12 b, illustrates current mirrors, in accordance with embodiments of the invention.

Corresponding numerals and symbols in the different figures generally refer to corresponding parts unless otherwise indicated. The figures are drawn to clearly illustrate the relevant aspects of the embodiments and are not necessarily drawn to scale.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The making and using of the presently preferred embodiments are discussed in detail below. It should be appreciated, however, that the present invention provides many applicable inventive concepts that can be embodied in a wide variety of specific contexts. The specific embodiments discussed are merely illustrative of specific ways to make and use the invention, and do not limit the scope of the invention.

The present invention will be described with respect to preferred embodiments in a specific context, namely as an asymmetric segmented channel transistor. Embodiments of the present invention include methods, structure, layouts, and circuits comprising asymmetric segmented channel transistor. The invention may also be applied, however, to other semiconductor components comprising, for example, other non CMOS devices. One of ordinary skill in the art will be able to recognize further examples as well.

Digital applications primarily drive CMOS scaling, in which the objectives are different—smaller, faster, and lower-power devices. Extremely scaled CMOS devices of about 30 nm are already in production, and further smaller devices are being explored. These small devices exhibit excellent speed (drive currents) but some of this performance is at the expense of much higher device off-current, about 100 nA/μm at 25° C., significantly compromising the low-standby-power attribute of CMOS technologies. To maintain enough gate current control, these extremely scaled devices typically use ultra-thin gate oxides (1.5 nm or thinner), resulting in gate tunneling current density as large as 10-100 A/cm². However, devices used in analog circuits cannot handle such large gate currents. Typically, this is not a problem as CMOS technologies offer multiple gate oxide technologies.

However, the speed (drive current) of the device is not the only Figure of Merit (FOM) the analog designer has to deal with. Other specifications that are as important if not more are device transconductance (g_(m)), output conductance (g_(ds)), cutoff frequency (f_(T)), maximum frequency of oscillation (f_(max)), Vt matching, 1/f noise and NF characteristics. Dynamic range is also important, along with other economic issues such as time to market, system specs and cost.

The small signal transconductance g_(m) is defined as change in drain current I_(ds) of a CMOS transistor for a given change in gate voltage V_(g) and hence g_(m)=dI_(ds)/dV_(g). The transconductance degrades with increase in gate voltage V_(g) due to degradation in mobility arising from increased surface roughness scattering. Again at high channel dopings, the transconductance degrades due to impurity induced coulomb (or impurity) scattering. In scaled CMOS devices, the channel doping is high due to the need for shielding source of the device from the drain of the device, or rather reduce the off current. Typically, halo implants are used to control short channel effects. However, this halo doping also counter-dopes the source and drain regions. Consequently the source and drain extension regions of the transistor are also significantly degraded (reducing g_(m)) due to increased resistance from the counter doping of the halo implants. A long channel transistor built with such halo implants may not be as significantly impacted by the reduced g_(m) around the source/drain regions as the middle section of the long channel transistor has no halo doping.

The parameter output conductance g_(ds) relates the sensitivity of drain current to a change in drain voltage, and hence g_(ds)=dI_(ds)/dV_(ds). A small output conductance is desirable for many analog circuit applications. However, high-speed CMOS devices usually have a large output conductance. The large output conductance is a direct result of the fact that short-channel devices have a large drain-induced barrier-lowering effect which causes the drain current to increase with drain voltage. Hence, as illustrated in FIG. 1, g_(ds) increases with gate length. When small output conductance is needed, designers often use device channel lengths significantly larger than minimum for the technology. However, the additional halo dose added to control the drain induced barrier lowering also significantly degrades the output conductance g_(ds). Hence, longer channel devices built on technologies optimized for short channel devices suffer from increased output conductance g_(ds).

The primary analog device design strives to: (a) maximize speed of the amplifier (˜g_(m)/C_(load)); (b) achieve g_(m) at lowest possible current to minimize power consumption (hence maximize g_(m)/I_(ds)); (c) maintain accuracy (e.g. of the single stage amplifier) by maximizing the voltage gain g_(m)/g_(ds); (d) minimize device mismatch (e.g. V_(T)) between identical devices; and (e) minimize noise (low frequency 1/f or flicker noise and high frequency noise such as NF noise).

A commonly used figure of merit for quantifying speed or g_(m)/C_(load) is the cutoff frequency (f_(T)), which is defined as the frequency at which the small signal current gain drops to unity. The cutoff frequency is a measure of the maximum useful frequency of a transistor when it is used as an amplifier. Scaled CMOS devices can have very high cutoff frequency. However, the series resistances associated with the source/drain and gate, together with the characteristic short-channel effects associated with very-short-channel CMOS devices, limits the attainable maximum frequency of oscillation (f_(max)). The maximum frequency of oscillation (f_(max)) is the frequency at which unilateral current gain becomes unity, and is hence a more reliable metric. The typical scale of operation of an application is about 5-10 times lower than the f_(T).

Currently there is great interest in developing CMOS with reduced parasitic resistance, including using a metal as the gate electrode. With a metal gate and careful layout to minimize parasitic resistance, the attainable f_(max) could approach 300 GHz. However, another key metric, intrinsic voltage gain g_(m)/g_(ds) is significantly degraded for scaled CMOS devices (FIG. 1).

FIG. 2 illustrates the tradeoff in key parameters in designing analog devices. For a given gate length device, a peak in f_(T) (curve 11) and f_(max) (curve 12). For example, f_(T) increases with source to drain current I_(ds) (or gate over drive). However, both g_(m)/g_(ds) (curve 14) and g_(m)/I_(ds) (curve 13) degrade with increasing source to drain current I_(ds). At any given technology node, simultaneous maximization of f_(T) and intrinsic voltage gain g_(m)/g_(ds) is not possible. Hence, analog devices are typically designed based on application i.e. high performance analog technologies favoring higher speed, and precision analog technology favoring intrinsic voltage gain. Hence, independently optimized devices are used to overcome these limitations. For example, devices with high intrinsic gain can be fabricated using long channel length with low channel doping. These devices may be suitably designed to maintain a high intrinsic voltage gain. However, as discussed this comes with a penalty in f_(T).

Hence, applications requiring high f_(T) may be custom designed on a different technology such as a BiCMOS process flow. However, rather than implement expensive solutions such as adopting an independent BiCMOS process flow, many applications require a low cost solution and instead compromise some of the device metrics. For example, a BiCMOS process may be 30-40% more expensive than a corresponding CMOS process. Hence, for many applications devices are tailored for each application and do not utilize the product development and enhancements available in CMOS process flows. For example, such analog devices are designed with thicker oxides, longer channel lengths, and unique source/drain extensions. Consequentially, this process requires a custom CMOS flow and is still expensive for many applications.

In various embodiments, the present invention overcomes these limitations by reducing the process complexity associated with formation of analog high performance and/or high precision CMOS devices. In various embodiments, the analog device comprises a combination of a short gate length CMOS device and a long gate CMOS device into a single structure to tailor an analog device with suitable device characteristics. In various embodiments, properties of a longer gate length device are achieved by a suitable combination of smaller gate length devices.

Structural embodiments of the invention will be first described using FIGS. 3 and 6. Various embodiments of the method of fabrication will then be described using the flow chart of FIG. 7, and FIG. 8. Embodiments of manufacturing and chips manufactured with logic and analog devices are illustrated in FIGS. 9 and 10. Finally, embodiments of the invention applied in analog circuits will illustrated in FIGS. 11 and 12.

An embodiment of the invention is illustrated in FIG. 3 which includes FIG. 3 a-3 c. FIGS. 3 a and 3 b illustrate the top cross sectional view and FIG. 3 c illustrates a vertical cross sectional view of the asymmetric channel transistor.

FIG. 3 a illustrates a top cross section of the device illustrating a layout (e.g. regions in mask layers for forming regions) in an embodiment of the invention. The device is embedded in an isolation region 40, which, for example, may be an STI region.

The silicon active region 20 is surrounded by the isolation region 40 and contains a source 30 and a drain 60. The gate line 50 includes a “U” shaped region forming two distinct gate regions overlying the silicon active region 20. The contacts 70, 75 and 80 are disposed on the drain 60, gate line 50 and source 30 respectively. FIG. 3 b illustrates the top cross section after formation of active regions. As illustrated in FIG. 3 b, the spacer 51 is disposed around the gate line 50. The gate line 50 and the spacer 51 overlie the isolation region 40 forming the “U” shaped region. FIG. 3 c illustrates the corresponding vertical cross section of the device 10 disposed in a substrate 100. The isolation region 40 of the device 10 comprises shallow trench isolation and typically comprises a trench filled with an oxide material. The source is contacted via source contact 80 by connecting to a voltage node and applies a first voltage. The second terminal or drain contact 70 is connected to voltage node to apply a second voltage. The gate line 50 is connected to a third voltage using gate contacts 75. The gate line 50 includes a first gate 90 and a second gate 95. The first and second gate 90 and 95 form corresponding first channel 91 and second channel 96 underneath. The potential difference applied between the source contact 80 and drain contact 70 determines the direction of the current flow (shown by the arrow). For the sake of illustration, a forward biased device has a current flow as shown by the arrow in FIG. 3 c. A reverse biased device has the potential difference reversed and hence reversing the direction of the corresponding flow of current. In all these cases the gate contacts 75 (and substrate 100) are biased to form an inversion layer in the first and second channels 91 and 96.

As illustrated in FIG. 3 a, the first gate 90 has a gate length L₁ and width W₁, and the second gate 95 has a gate length L₂ and width W₂. In various embodiments, the first gate 90 and second gate 95 comprise different gate lengths. In preferred embodiments, the second gate length L₂ is about 1.5 to 2.5 times the length of the first gate length L₁, and the first gate length L₁ is at the minimum gate length for the given technology. Typically but necessarily, the widths W₁ and W₂ are about equal.

In various embodiments, a method of optimization for determining the lengths and widths uses an optimization procedure to match the target specifications derived for an individual circuit. The device is tailored for each circuit by individually changing the lengths and widths of the devices.

The performance of the device 10 (e.g. of FIG. 3 c) after the optimization, in different embodiments is illustrated in FIGS. 4 and 5. FIGS. 4 a-4 c illustrate a precision analog device with optimized intrinsic voltage gain. Curve 2 refers to the device in FIG. 1 biased in forward bias mode. Curve 3 refers to the device in FIG. 1 biased in reverse bias mode. Curve 1 refers to a standard symmetric device. The standard symmetric device is a longer channel device such that the gate length of the standard device is longer than the sum of the first gate length L₁ of the first gate and the second gate length L₂ of the second gate. Using embodiments of the invention, curve 2 illustrates improvements in g_(m) and g_(ds) (hence, the ratio g_(m)/g_(ds)) obtainable over the standard device especially in bias conditions suitable for analog operation (gate voltage is about 100 mV above the threshold voltage). Even the reverse biased device shows respectable device characteristics.

FIG. 5 illustrates the embodiments of the invention applied for producing a analog device for high performance applications. In such applications, a high f_(T) is required while maximizing the intrinsic gain. Hence, FIG. 6 illustrates g_(m)/g_(ds) at constant f_(T). In FIG. 6, curve 1 represents a long channel device and curve 4 illustrates a short channel device showing degraded g_(m)/g_(ds). Curve 2 illustrates the forward biased asymmetric device illustrating an improved intrinsic voltage gain, using embodiments of the invention e.g. as shown in FIG. 3. Curve 3 illustrates the reverse biased asymmetric device.

FIG. 6, which includes FIGS. 6 a-6 e, illustrates various structural embodiments of the invention. In FIGS. 6 a and 6 d, the gate line 50 includes three gates. Hence, the device 10 includes a first gate 90, a second gate 95 and a third gate 105. Each gate length is individually optimized. In FIG. 6 b, the gate line 50 shares two active areas 110 and 120 forming two transistors with their own source 30 and drain 60 regions. FIG. 6 c illustrates an embodiment, wherein the first gate 90 and second gate 95 are connected from both ends. FIG. 6 e illustrates an embodiment in which the both the gate lengths and widths of the gate lines are individually optimized. Although in this embodiment, the section with the larger gate length has a smaller width, other embodiments may use a different combination. For example, the section with the larger gate length may have a larger width as illustrated in FIG. 6 f. The larger width may be preferable in some embodiments, as the higher total current flowing through the larger width transistor results in a decrease in output conductance g_(ds) of the shorter width transistor. Consequently improving the intrinsic voltage gain g_(m)/g_(ds) of the combined devices.

A method of fabrication is illustrated in flow chart of FIG. 7 along with cross sectional views in FIG. 8, in accordance with an embodiment of the invention.

The analog device flow essentially comprises changing a digital technology flow to accommodate the analog device features. Hence, only some processes pertaining to the analog device will be illustrated in FIGS. 7 and 8.

FIG. 8 a is a top cross sectional view and FIG. 8 b is a vertical cross section. Referring first to FIGS. 8 a and 8 b and the flow chart of FIG. 7, a semiconductor substrate 100 is provided. In the preferred embodiment, the semiconductor substrate 100 is a silicon wafer. Some examples of the semiconductor substrate 100 are a bulk mono-crystalline silicon substrate (or a layer grown thereon or otherwise formed therein), a layer of a silicon-on-insulator (SOI) wafer.

Isolation trenches 28 are formed in the semiconductor substrate 100 using conventional techniques. The isolation trenches 28 are then filled with an isolating material forming isolation 40. The isolation process defines the widths W of the transistors (to be formed).

The well implants, isolation implants, and threshold adjust implants are next implanted. The analog regions are masked separately and implanted separate from the logic regions. Some of the implants may be tailored to minimize noise from other active regions. For example, noise from other active devices may be transferred through the substrate. These signals could be generated anywhere in the chip, for example in regions where the drains and collectors are capacitively coupled to the substrate. These noises could be shielded by adding sufficient isolation regions. In various embodiments, the isolation implants could be modified for the analog device to shield the device from such external noise.

Referring next to FIG. 8 c, a gate dielectric 24 is deposited over exposed portions of the semiconductor substrate 100. Advanced deeply scaled CMOS nodes have considerable gate leakage due to the use of ultra-thin gate dielectrics. For example, gate leakage can be of the order of 1-10 nA/μm for thin gate oxides. Analog circuits cannot tolerate such high gate leakages. For low-noise analog circuits, the gate leakage current level should be controlled. CMOS devices near the scaling limit are not suitable for low noise analog circuits. Also, analog circuit designers prefer to design with power-supply voltages much larger than that of high-speed digital CMOS. However, most CMOS nodes include multiple gate oxide technologies especially for designing high voltage transistors such as Input/Output (I/O) devices. A thick gate oxide is chosen from available oxides to minimize gate tunneling leakage. Of course, the thick-oxide devices are slower than the thin-oxide ones. In one embodiment, the gate dielectric 24 comprises an oxide (e.g., SiO₂), a nitride (e.g., Si₃N₄), or a combination of oxide and nitride (e.g., SiON, or an oxide-nitride-oxide sequence).

The gate electrode layer 26 is deposited over the gate dielectric 24. The gate electrode 26 preferably comprises a semiconductor material, such as polysilicon or amorphous silicon, although alternatively, other semiconductor materials or metal may be used. The gate electrode layer 26 having a thickness of between about 400 Å to 2000 Å may be deposited using CVD, PVD, ALD, or other deposition techniques.

A resist layer 311 is deposited over the gate electrode layer 26. The resist layer 311 may be either organic or inorganic. Some examples of inorganic resist layer 311 include silicon dioxide, silicon nitride, silicon oxy-nitride, titanium nitride and/or a SILK (silicon-containing low-k) layer. The resist layer 311 may also be an organic layer such as a bottom anti-reflective coating (BARC) layer (such as polymides, and polysulfones), a FLARE layer, and/or a BCB layer. The resist layer 311 may optionally be baked to form a hard baked, thermally or chemically cross-linked resist. Finally, although only a single layer of resist 311 is shown, the resist layer 311 may comprise multiple layers. For example, in some embodiments, the resist layer 311 may be a bilayer or trilayer film comprising different materials.

A photo-resist 315 is deposited on the gate stack. The photo-resist layer 315 is a resist that can be developed by exposure to radiation such as deep UV radiation used by lithography systems. In preferred embodiments, this photo-resist 315 is sensitive to 243 nm, 193 nm or 157 nm electromagnetic radiation. The resist used may either be positive or negative. Examples of resist polymers are poly-p-hydroxystyrene, acrylates, novolak or cycloaliphatic copolymers.

Referring to FIG. 8 d, the photo-resist 315 is exposed using a mask level comprising the desired gate features (first and second gates 90 and 95). The exposed photo-resist regions are removed by etching. Using the remaining photo-resist regions as patterns, an anisotropic etch such as a reactive ion etch is used to remove the exposed portion of the resist 311. The vertical cross section at this stage of the fabrication process is illustrated in FIG. 8 e. Subsequent processing proceeds as in a conventional CMOS flow, forming drain extensions, source/drains using anneals such a spike and/or laser anneals.

Embodiments of the invention will now be described to illustrate applications of the invention in manufacturing an integrated mixed signal system on chip (SoC) integrated circuits. SoC comprise both analog and digital components. As detailed in various embodiments above both the digital and analog components are manufactured using a common process. Embodiments illustrated in FIGS. 9 and 10 describe the final SoC manufactured using such a common process.

Referring to FIG. 9 a, the SoC device 11 comprises digital and analog devices. The digital devices include CMOS devices. For example, in FIG. 9 a, the digital devices include a nMOS 207 and a pMOS 208. The SoC device 11 also includes the analog device 10. The nMOS 20, the pMOS 208 and the analog device 10 are formed in a substrate 100 and comprise source 30, drain 60, separated by gate line 50 and spacers 51. The nMOS 207 comprises a p-channel 92 and the pMOS 208 comprises an n-channel 93. The analog device 10 includes a first transistor 900 and a second transistor 950. First channel 91 and second channel 96 are formed in the first transistor 900 and a second transistor 950 respectively. The source of the first transistor 900 is connected to the source 30; and the drain of the second transistor 950 is connected to the drain 60. The drain of the first transistor 900 and the source of the second transistor 950 are connected via the shared source/drain 31. The potential difference applied between the source contact 80 and drain contact 70 determines the direction of the current flow (shown by the arrows).

In various embodiments of the invention, the threshold voltage of the first transistor 900 is higher compared to a threshold voltage of the second transistor 950. When the drain 60 of the device 10 is biased using the contact 70, the lower threshold voltage of the second transistor 950 results in an increased potential on the shared source/drain 31. Consequently, the first transistor 900 sees a higher drain voltage resulting in a decrease in output conductance g_(ds) of the first transistor 900 reducing the overall output conductance g_(ds) of the combined first and second transistors 900 and 950. The reduced overall output conductance g_(ds) of the combined first and second transistors 900 and 950 translates into a higher analog gain.

In various embodiments, the nMOS 20, the pMOS 208 and the analog device 10 are manufactured preferably using the same process, for example, as described in FIGS. 7 and 8. In different embodiments, the difference in threshold voltages between first and second transistors 900 and 950 is obtained by a difference in gate lengths between the transistors. In various embodiments, this difference may also be obtained by using the high voltage or low voltage oxide, or alternately the low Vt or high Vt drain extension (e.g., halo) implants. Note that neither of these processes require additional masking steps. E.g. process flow for logic manufacturing already includes multiple gate oxides, for example, for I/O devices and core devices. The high voltage oxide for forming I/O devices can be used for the first transistor 900 and the low voltage oxide for forming the core devices can be used for the second transistor 950. Similarly, different Vt masks are already in use in logic processing. A higher halo dose (e.g., used in the high Vt cmos device) can be used in forming the first transistor 900 and low halo dose (e.g. used in low Vt or nominal Vt cmos device) can be used in forming the second transistor 950.

The threshold voltage of the first transistor 900 is preferably about 200 mV higher than the threshold voltage of the second transistor 950. In various embodiments, the threshold voltage of the first transistor 900 is at least about 100 mV higher than the threshold voltage of the second transistor 950. Although in other embodiments, the threshold voltage of the first transistor 900 may be about 100 mV to about 300 mV higher than the threshold voltage of the second transistor 950.

FIG. 9 b illustrates threshold voltage of a transistor (V_(T)) versus inverse gate length (1/L) of devices manufactured simultaneously using a given CMOS process. For example, the digital devices (nMOS 207 and pMOS 208) in FIG. 9 a may be formed using the shorter channel devices, and typically are formed around the minimum gate length device. As illustrated in FIG. 9 b, the first transistor 900 with a higher threshold voltage may be selected from short channel devices, whereas the second transistor 950 with the lower threshold voltage may be selected from long channel devices. The first transistor 900 is preferably positioned around the peak of the threshold voltage versus 1/L curve. FIG. 9 b illustrates the absolute threshold voltage. Hence, if both devices are pFET's, the first transistor 900 is more negative than the second transistor 950.

FIG. 10, which includes FIGS. 10 a and 10 b illustrates a SoC implementing multiple gate analog devices. Unlike FIG. 9 a, in FIG. 10 a the analog device 10 comprises three transistors: a first transistor 900, a second transistor 950, and a third transistor 1050. As in the previous embodiments, the threshold voltage of the first transistor 900 is greater than the threshold voltage of the second transistor 950. Also, the threshold voltage of the second transistor 950 is greater than the threshold voltage of the third transistor 1050. One way of implementing the transistors with different threshold voltages is by using transistors of different gate lengths. In this case, care should be taken that the first transistor 900 is not made so short that its threshold voltage becomes lower than the second transistor 950 due to short channel effects. For example, as illustrated in FIG. 10 b, the range of the first transistor 900 may be minimized or centered around the peak of the threshold voltage rolloff. In various embodiments, the first transistor 900 may be larger than the minimum allowed gate length (L_(min)) or the gate length at which most of the digital devices are manufactured. Although not discussed, in various embodiments, the analog device 10 may comprise more than three transistors.

Embodiments of the invention will now be described to illustrate applications of the invention to analog circuits in FIGS. 11 and 12. FIG. 11, which includes FIGS. 11 a-11 c, illustrates op-amp circuits incorporating embodiments of the invention. FIG. 12, which includes FIG. 12 a and FIG. 12 b, illustrates current minors, in accordance with embodiments of the invention. Although, only a few analog circuits are described here, embodiments of the invention apply to all analog circuits.

FIG. 11 a illustrates an effective circuit, and FIGS. 11 b-11 c illustrate the complete circuit for a two-stage CMOS op-amp, in accordance with an embodiment of the invention. The circuit consists of an input differential stage with active load followed by a common-source stage, also with active load. The amplifier is required to drive a fixed on-chop capacitive load of a few picofarads. This amplifier is a typical op-amp used in a large number of semiconductor components. For example, it is used applications in switched capacitor filters, sensing circuits, analog to digital converters etc.

In one embodiment, the op-amp is designed by optimizing the effective circuit of FIG. 11 a. For example, the widths (W₁-W₈) and lengths (L₁-L₈) of the individual transistors (M₁-M₈), as well as the compensation capacitor C_(c) and resistance R_(c), and bias current I_(b) may be optimized. The design parameters also include the voltages V_(SS) and V_(DD). During analog device design the widths (W₁-W₈) and lengths (L₁-L₈) are determined. However, the optimized individual transistors (M₁-M₈) may require very high analog gains unachievable using a shared CMOS process. Alternately, in some embodiments, the optimized large gate lengths may be larger than the maximum allowed gate length in a CMOS technology.

FIG. 11 b illustrates an embodiment of the invention wherein each of the transistors in FIG. 11 a is replaced by two or more transistors of smaller gate lengths. Hence, the transistors M₁-M₈ comprises transistors (M₁₁, M₁₂)-(M₈₁, M₈₂) with corresponding gate lengths (L₁₁, L₁₂)-(L₈₁, L₈₂). The gate lengths (L₁₁, L₁₂)-(L₈₁, L₈₂) are smaller than the corresponding gate lengths of the single gate transistors illustrated in FIG. 11 a. The sets of transistors, for example, (M₁₁, M₁₂) representing M₁, together match an expected analog voltage gain, a goal previously unobtainable without altering the manufacturing process.

Although, in FIG. 11 b, all of the transistors in FIG. 11 a are replaced with two smaller transistors, other embodiments may replace only some of the transistors. For example, after analog design, only some of the transistors may require high analog voltage gain. For example, a voltage gain of about 50 or higher may be required by only some devices. In such embodiments, only those transistors are replaced with two or more transistors. For example, in FIG. 11 c, only the transistor M₈ of FIG. 11 a is replaced with transistors M₈₁₁, M₈₂₁ and M₈₃₁.

FIG. 12 a illustrates an effective current mirror, and FIG. 12 b illustrates the complete current mirror, in accordance with embodiments of the invention. The effective current minor illustrated in FIG. 10 a includes effective transistors M₁₁, and M₁₃, and transistor M₁₂. The transistor M₁₂ has a shorter gate length L₁₂ than the gate length L₁₁ of the other two effective transistors (M₁₁ and M₁₃).

FIG. 12 b illustrates the complete current mirror of FIG. 12 a. As the analog voltage gain of the transistor M₁₁ with gate length L₁₁ is high, the transistor M₁₁ is replaced with other two transistors (M₁₁₁ and M₁₁₂) with an effective larger voltage gain. Similarly, if the required voltage gain of the transistor M₁₃ with gate length L₁₁ is high, the transistor M₁₃ is replaced with other two transistors (M₁₃₁ and M₁₃₂). In various embodiments, the effective transistors M₁₁ and M₁₃ are divided into transistors with gate lengths L₁₁₁ and L₁₁₂, wherein the transistor M₁₃₂ has a lower threshold voltage than the transistor M₁₃₁. In some embodiments, the gate length L₁₁₂ of the transistor M₁₃₂ is larger than the gate length L₁₁₁ of the transistor M₁₁₁. The gates of the transistors M₁₁₁ and M₁₁₂ are tied together forming the effective transistor M₁₁. Similarly, the gates of the transistors M₁₃₁ and M₁₃₂ are tied together forming the effective transistor M₁₃. The source/drain of the transistors M₁₁₂ and M₁₃₂ are tied together forming the current mirror circuit.

Although embodiments of the present invention and their advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims. For example, it will be readily understood by those skilled in the art that many of the features, functions, processes, and materials described herein may be varied while remaining within the scope of the present invention.

Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed, that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps. 

What is claimed is:
 1. A method of designing an integrated circuit, the method comprising: designing a first transistor comprising a first threshold voltage, a first gate length and a first width; designing a second transistor comprising a second threshold voltage, a second gate length and a second width, wherein the first gate length is smaller than the second gate length, wherein the first threshold voltage is higher than the second threshold voltage; and designing an asymmetric transistor by combining the first transistor and the second transistor, wherein the combining comprises electrically coupling a drain of the first transistor and a source of the second transistor and electrically coupling a gate of the first transistor with a gate of the second transistors, wherein, under a forward bias, the asymmetric transistor has a voltage gain higher than a symmetric long channel transistor having a gate length longer than a sum of the first and the second gate lengths.
 2. The method of claim 1, further comprising using the asymmetric transistor in designing a current mirror circuit.
 3. The method of claim 1, further comprising using the asymmetric transistor in designing a amplifier circuit.
 4. The method of claim 1, further comprising using the asymmetric transistor in designing a two-stage CMOS op-amplifier circuit.
 5. The method of claim 1, wherein, under a reverse bias, the asymmetric transistor has a voltage gain comparable to the symmetric long channel transistor.
 6. The method of claim 1, wherein the drain of the first transistor and the source of the second transistor share a common region, and wherein the gate of the first transistor is physically coupled to the gate of the second transistor.
 7. The method of claim 1, further comprising a gate dielectric layer disposed under the gate, wherein a portion of the gate dielectric layer under the first transistor is identical to a portion of the gate dielectric layer under the second transistor.
 8. The method of claim 1, wherein the first and second transistors are of a same type.
 9. The method of claim 1, wherein a thickness of a gate dielectric of the first transistor is the same as a thickness of a gate dielectric of the second transistor.
 10. The method of claim 1, wherein the integrated circuit is manufactured in a CMOS process common to digital devices.
 11. The method of claim 1, wherein designing an asymmetric transistor comprises: calculating an output analog voltage gain of the asymmetric transistor; determining an error between the calculated output analog voltage gain and a target analog voltage gain; and redesigning the first threshold voltage and a second threshold voltage to lower the error below an acceptable level.
 12. A method of designing an integrated circuit, the method comprising: designing a first transistor comprising a first threshold voltage, a first gate length and a first width; designing a second transistor comprising a second threshold voltage, a second gate length and a second width, wherein the first gate length is smaller than the second gate length, wherein the first threshold voltage is higher than the second threshold voltage; and designing an asymmetric transistor for a current mirror circuit within the integrated circuit by combining the first transistor and the second transistor, wherein the combining comprises electrically coupling a drain of the first transistor and a source of the second transistor and electrically coupling a gate of the first transistor with a gate of the second transistors, wherein, under a forward bias, the asymmetric transistor has a voltage gain higher than a symmetric long channel transistor having a gate length longer than a sum of the first and the second gate lengths.
 13. The method of claim 12, wherein the drain of the first transistor and the source of the second transistor share a common region, and wherein the gate of the first transistor is physically coupled to the gate of the second transistor.
 14. The method of claim 12, wherein the first transistor and the second transistor comprise p-channel field effect transistors.
 15. The method of claim 12, wherein the first transistor and the second transistor comprise n-channel field effect transistors.
 16. The method of claim 12, wherein, under a reverse bias, the asymmetric transistor has a voltage gain comparable to the symmetric long channel transistor.
 17. A method of designing an integrated circuit, the method comprising: designing a first transistor having a channel of a first conductivity type, the first transistor comprising a first threshold voltage, a first gate length and a first width; designing a second transistor having a channel of the first conductivity type, the second transistor comprising a second threshold voltage, a second gate length and a second width, wherein the first gate length is smaller than the second gate length, wherein the first threshold voltage is higher than the second threshold voltage; and designing an asymmetric transistor for an amplifier circuit within the integrated circuit by combining the first transistor and the second transistor, wherein the combining comprises electrically coupling a drain of the first transistor and a source of the second transistor and electrically coupling a gate of the first transistor with a gate of the second transistors, wherein, under a forward bias, the asymmetric transistor has a voltage gain higher than a symmetric long channel transistor having a gate length longer than a sum of the first and the second gate lengths.
 18. The method of claim 17, wherein the drain of the first transistor and the source of the second transistor share a common region, and wherein the gate of the first transistor is physically coupled to the gate of the second transistor.
 19. The method of claim 17, wherein, under a reverse bias, the asymmetric transistor has a voltage gain comparable to the symmetric long channel transistor. 